The broad objective of this project is to describe the brain's specializations for sound localization. The initial specific aim of the present application is to evaluate the ascending afferent representation of spectrum-based localization cues that are generated by directional properties of the head and outer ear. These filtering effects are known as the head-related transfer function (HRTF). It is hypothesized that this complex acoustic information is encoded by the discharge rates of a select population of inferior colliculus neurons, the type O units. Subsequent aims will expand upon this hypothetical construct of a functionally segregated spectral processing pathway to determine how neural representations of the HRTF are enhanced by descending efferent influences and learned listening strategies. Electrophysiological experiments will record single-unit responses to virtual space stimuli that simulate the natural directional properties of the HRTF with headphones. Related psychophysical experiments will confirm the functional significance of observed physiological mechanisms by measuring the effects of spectral pathway lesions on sound localization behavior. Electrophysiological studies in behaviorally conditioned cats will explore the mechanisms for adaptive listening under training contingencies that bias the information content of individual features of the HRTF. The spectral shapes that contribute to sound localization are closely related to the sounds of human speech. Consequently, this research will enhance our current understanding of the processes and perceptions that shape directional hearing, speech processing, and compensation for hearing loss in humans. [unreadable] [unreadable]